Field of the Invention
The present invention relates to a process for preparing a cathode active material for a lithium secondary battery. More particularly, it relates to a process for preparing a cathode active material for providing lithium secondary batteries with improved performance in a markedly shortened time.
With the recent rapid development of portable and wireless electronic equipment such as personal computers and telephones, the demand for secondary batteries as a driving power source has been increasing. In particular lithium secondary batteries are expected for their smallest size and high energy density. Cathode active materials for lithium secondary batteries meeting the demand include lithium cobaltate (LiCoO.sub.2), lithium nickelate (LiNiO.sub.2), and lithium manganate (LiMn.sub.2 O.sub.4). Having an electrode potential of 4 V or higher with respect to lithium, these lithium complex oxides provide lithium secondary batteries having a high energy density.
Compared with LiNiO.sub.2 and LiCoO.sub.2 having a theoretical capacity of about 280 mAh/g, LiMn.sub.2 O.sub.4 has a theoretical capacity as low as 135 mAh/g but is deemed suited for use in electric vehicles because of an abundant and inexpensive supply of manganese oxide as a raw material and freedom from such thermal instability in charging as observed with LiNiO.sub.2.
Thackeray et al. teach a process for synthesizing LiMn.sub.2 O.sub.4 which comprises mixing Mn.sub.2 O.sub.3 and Li.sub.2 CO.sub.3 at a molar ratio of 2:1 and firing the mixture at 650.degree. C. for 12 hours and then at 850.degree. C. for 24 hours (see Mat. Res. Bull., Vol. 18, pp. 461-472 (1983)). Tarascon et al. propose mixing Li.sub.2 CO.sub.3 and MnO.sub.2 and firing the mixture at 600 to 1100.degree. C. for 48 hours (see J. Electrochem. Soc., Vol. 138, No. 10, pp. 2859-2864 (1991)). These conventional processes are disadvantageous in that the firing time is long, thereby increasing the cost for industrial mass production.
Japanese Patent Laid-Open No. 129229/97 discloses a process for preparing lithium cobaltate, etc. useful as a cathode active material of lithium secondary batteries, in which the rate of temperature drop in the firing step is set at 300.degree. C. min or higher. However, the disclosed technique does not apply to the production of lithium manganate because the reaction mechanism for lithium cobaltate formation and that for lithium manganate differ. More specifically, with respect to raw materials, for example, the publication mentions that materials having a low melting or decomposition point are suitable and, in fact, Ni(OH).sub.2 and LiOH are used in all the working Examples. However, LiOH is unsuitable as a raw material for LiMn.sub.2 O.sub.4, and Li.sub.2 CO.sub.3 that is suitable as a raw material for LiMn.sub.2 O.sub.4 has a decomposition point of 720.degree. C. or lower, which is considerably higher than those of Ni(OH).sub.2 (about 300.degree. C.) or LiOH (420.degree. C.). Further, Ni or Co oxide is an unsuitable material for generation of LiNiO.sub.2 -based compounds, while MnO.sub.2 is suited as a raw material of LiMn.sub.2 O.sub.4.